Abstract 5973: The brain penetrant CDK4/6 Inhibitor, PRT3645, is highly effective in combination with other targeted therapies in preclinical models of NSCLC, CRC, and HER2-positive breast cancer

Abstract Cell cycle deregulation is a hallmark of cancer and the hyperactivation and overexpression of CDKs are often drivers of cancer pathogenesis. Cyclin-dependent kinase 4 and 6 (CDK4)/(CDK6) are critical mediators of cellular transition into S phase and important for the initiation, growth, and survival of many cancers. Activated CDK4/CDK6 complexes phosphorylate Rb1, reduce their binding affinities and release Rb1-containing transcription repressor complexes from E2F transcription factors, resulting in activation of E2F controlled cell cycle genes and progression of the cell cycle. At present three CDK4/CDK6 inhibitors are approved for the treatment of ER+/HER2- breast cancer, and are being explored in other cancer indications as well. Previously we described a novel brain penetrant CDK4/CDK6 inhibitor, PRT3645, that exhibits single digit nanomolar biochemical potency against CDK4/CDK6 and >2000-fold selectivity against CDK1, CDK2 and CDK9. PRT3645 inhibits cellular phosphorylation of Rb and exhibits a protein binding-adjusted cellular IC50 of <300 nM. PRT3645 exhibits favorable in-vitro safety pharmacology and ADME properties, including increased brain penetration, and demonstrates oral bioavailability across rodents, dog, and non-human primates. In addition to robust monotherapy activity observed in preclinical models of ER+/HER2- breast cancer, we explored the activity of PRT3645 in other tumor types as well as in combination with other targeted therapies. In NSCLC, PRT3645 treatment resulted in significant inhibition of cell lines that harbor activation of the RAS/MEK/ERK pathway in proliferation assays and demonstrated comparable high synergy scores when combined with clinically approved covalent KRAS G12C inhibitors. In-vivo, oral PRT3645 was well tolerated and induced anti-tumor efficacy in two KRAS G12C mutant xenograft models that harbor the CDKN2A (p16) deletion. Anti-tumor efficacy was further improved when PRT3645 was combined with KRAS/MEK inhibitors in xenograft models and the combination therapy was well tolerated. In addition, we explored combinations of PRT3645 with a brain penetrant receptor tyrosine kinase inhibitor (TKI), an approved treatment for patients with advanced HER2+ breast cancer, including patients with brain metastases. In a HER2+ orthotopic human breast cancer brain metastasis model, PRT3645 was highly efficacious in combination with HER2 kinase inhibition and enhanced median survival significantly. In summary, PRT3645 demonstrates an excellent balance of potency, selectivity, PK parameters across species and brain penetrance. In preclinical studies, PRT3645 was highly efficacious when combined with KRAS/MEK inhibitors, and with a brain penetrant HER2 receptor TKI, both in-vitro and in-vivo. PRT3645 has advanced into Phase 1 clinical trials (NCT05538572). Citation Format: Yue Zou, Srijita Dhar, Kirsten Gallagher, Andrew Buesking, Sarah Pawley, Ryan Holmes, Xiaowei Wu, Katarina Rohlfing, Min Wang, Joseph Rager, Tom Emm, Stefan Ruepp, Miles Cowart, Jing Ni, Jean Zhao, Bruce Ruggeri, Andrew Combs, Kris Vaddi, Sandy Geeganage, Ashish Juvekar, Sang Hyun Lee, Peggy Scherle. The brain penetrant CDK4/6 Inhibitor, PRT3645, is highly effective in combination with other targeted therapies in preclinical models of NSCLC, CRC, and HER2-positive breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5973.